The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/Documentation/initrd.txt

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    1 Using the initial RAM disk (initrd)
    2 ===================================
    3 
    4 Written 1996,2000 by Werner Almesberger <werner.almesberger@epfl.ch> and
    5                      Hans Lermen <lermen@fgan.de>
    6 
    7 
    8 initrd provides the capability to load a RAM disk by the boot loader.
    9 This RAM disk can then be mounted as the root file system and programs
   10 can be run from it. Afterwards, a new root file system can be mounted
   11 from a different device. The previous root (from initrd) is then moved
   12 to a directory and can be subsequently unmounted.
   13 
   14 initrd is mainly designed to allow system startup to occur in two phases,
   15 where the kernel comes up with a minimum set of compiled-in drivers, and
   16 where additional modules are loaded from initrd.
   17 
   18 This document gives a brief overview of the use of initrd. A more detailed
   19 discussion of the boot process can be found in [1].
   20 
   21 
   22 Operation
   23 ---------
   24 
   25 When using initrd, the system typically boots as follows:
   26 
   27   1) the boot loader loads the kernel and the initial RAM disk
   28   2) the kernel converts initrd into a "normal" RAM disk and
   29      frees the memory used by initrd
   30   3) initrd is mounted read-write as root
   31   4) /linuxrc is executed (this can be any valid executable, including
   32      shell scripts; it is run with uid 0 and can do basically everything
   33      init can do)
   34   5) linuxrc mounts the "real" root file system
   35   6) linuxrc places the root file system at the root directory using the
   36      pivot_root system call
   37   7) the usual boot sequence (e.g. invocation of /sbin/init) is performed
   38      on the root file system
   39   8) the initrd file system is removed
   40 
   41 Note that changing the root directory does not involve unmounting it.
   42 It is therefore possible to leave processes running on initrd during that
   43 procedure. Also note that file systems mounted under initrd continue to
   44 be accessible.
   45 
   46 
   47 Boot command-line options
   48 -------------------------
   49 
   50 initrd adds the following new options:
   51 
   52   initrd=<path>    (e.g. LOADLIN)
   53 
   54     Loads the specified file as the initial RAM disk. When using LILO, you
   55     have to specify the RAM disk image file in /etc/lilo.conf, using the
   56     INITRD configuration variable.
   57 
   58   noinitrd
   59 
   60     initrd data is preserved but it is not converted to a RAM disk and
   61     the "normal" root file system is mounted. initrd data can be read
   62     from /dev/initrd. Note that the data in initrd can have any structure
   63     in this case and doesn't necessarily have to be a file system image.
   64     This option is used mainly for debugging.
   65 
   66     Note: /dev/initrd is read-only and it can only be used once. As soon
   67     as the last process has closed it, all data is freed and /dev/initrd
   68     can't be opened anymore.
   69 
   70   root=/dev/ram0   (without devfs)
   71   root=/dev/rd/0   (with devfs)
   72 
   73     initrd is mounted as root, and the normal boot procedure is followed,
   74     with the RAM disk still mounted as root.
   75 
   76 
   77 Installation
   78 ------------
   79 
   80 First, a directory for the initrd file system has to be created on the
   81 "normal" root file system, e.g.
   82 
   83 # mkdir /initrd
   84 
   85 The name is not relevant. More details can be found on the pivot_root(2)
   86 man page.
   87 
   88 If the root file system is created during the boot procedure (i.e. if
   89 you're building an install floppy), the root file system creation
   90 procedure should create the /initrd directory.
   91 
   92 If initrd will not be mounted in some cases, its content is still
   93 accessible if the following device has been created (note that this
   94 does not work if using devfs):
   95 
   96 # mknod /dev/initrd b 1 250 
   97 # chmod 400 /dev/initrd
   98 
   99 Second, the kernel has to be compiled with RAM disk support and with
  100 support for the initial RAM disk enabled. Also, at least all components
  101 needed to execute programs from initrd (e.g. executable format and file
  102 system) must be compiled into the kernel.
  103 
  104 Third, you have to create the RAM disk image. This is done by creating a
  105 file system on a block device, copying files to it as needed, and then
  106 copying the content of the block device to the initrd file. With recent
  107 kernels, at least three types of devices are suitable for that:
  108 
  109  - a floppy disk (works everywhere but it's painfully slow)
  110  - a RAM disk (fast, but allocates physical memory)
  111  - a loopback device (the most elegant solution)
  112 
  113 We'll describe the loopback device method:
  114 
  115  1) make sure loopback block devices are configured into the kernel
  116  2) create an empty file system of the appropriate size, e.g.
  117     # dd if=/dev/zero of=initrd bs=300k count=1
  118     # mke2fs -F -m0 -b 1024 initrd
  119     (if space is critical, you may want to use the Minix FS instead of Ext2)
  120     (Note that due to a problem elsewhere in the kernel, you _must_ use a
  121      1024-byte blocksize when creating your file system.  If any other
  122      value is used, the kernel will be unable to mount the initrd at boot
  123      time, causing a kernel panic.)
  124  3) mount the file system, e.g.
  125     # mount -t ext2 -o loop initrd /mnt
  126  4) create the console device (not necessary if using devfs, but it can't
  127     hurt to do it anyway):
  128     # mkdir /mnt/dev
  129     # mknod /mnt/dev/console c 5 1
  130  5) copy all the files that are needed to properly use the initrd
  131     environment. Don't forget the most important file, /linuxrc
  132     Note that /linuxrc's permissions must include "x" (execute).
  133  6) correct operation the initrd environment can frequently be tested
  134     even without rebooting with the command
  135     # chroot /mnt /linuxrc
  136     This is of course limited to initrds that do not interfere with the
  137     general system state (e.g. by reconfiguring network interfaces,
  138     overwriting mounted devices, trying to start already running demons,
  139     etc. Note however that it is usually possible to use pivot_root in
  140     such a chroot'ed initrd environment.)
  141  7) unmount the file system
  142     # umount /mnt
  143  8) the initrd is now in the file "initrd". Optionally, it can now be
  144     compressed
  145     # gzip -9 initrd
  146 
  147 For experimenting with initrd, you may want to take a rescue floppy and
  148 only add a symbolic link from /linuxrc to /bin/sh. Alternatively, you
  149 can try the experimental newlib environment [2] to create a small
  150 initrd.
  151 
  152 Finally, you have to boot the kernel and load initrd. Almost all Linux
  153 boot loaders support initrd. Since the boot process is still compatible
  154 with an older mechanism, the following boot command line parameters
  155 have to be given:
  156 
  157   root=/dev/ram0 init=/linuxrc rw
  158 
  159 if not using devfs, or
  160 
  161   root=/dev/rd/0 init=/linuxrc rw
  162 
  163 if using devfs. (rw is only necessary if writing to the initrd file
  164 system.)
  165 
  166 With LOADLIN, you simply execute
  167 
  168      LOADLIN <kernel> initrd=<disk_image>
  169 e.g. LOADLIN C:\LINUX\BZIMAGE initrd=C:\LINUX\INITRD.GZ root=/dev/ram0
  170        init=/linuxrc rw
  171 
  172 With LILO, you add the option INITRD=<path> to either the global section
  173 or to the section of the respective kernel in /etc/lilo.conf, and pass
  174 the options using APPEND, e.g.
  175 
  176   image = /bzImage
  177     initrd = /boot/initrd.gz
  178     append = "root=/dev/ram0 init=/linuxrc rw"
  179 
  180 and run /sbin/lilo
  181 
  182 For other boot loaders, please refer to the respective documentation.
  183 
  184 Now you can boot and enjoy using initrd.
  185 
  186 
  187 Changing the root device
  188 ------------------------
  189 
  190 When finished with its duties, linuxrc typically changes the root device
  191 and proceeds with starting the Linux system on the "real" root device.
  192 
  193 The procedure involves the following steps:
  194  - mounting the new root file system
  195  - turning it into the root file system
  196  - removing all accesses to the old (initrd) root file system
  197  - unmounting the initrd file system and de-allocating the RAM disk
  198 
  199 Mounting the new root file system is easy: it just needs to be mounted on
  200 a directory under the current root. Example:
  201 
  202 # mkdir /new-root
  203 # mount -o ro /dev/hda1 /new-root
  204 
  205 The root change is accomplished with the pivot_root system call, which
  206 is also available via the pivot_root utility (see pivot_root(8) man
  207 page; pivot_root is distributed with util-linux version 2.10h or higher
  208 [3]). pivot_root moves the current root to a directory under the new
  209 root, and puts the new root at its place. The directory for the old root
  210 must exist before calling pivot_root. Example:
  211 
  212 # cd /new-root
  213 # mkdir initrd
  214 # pivot_root . initrd
  215 
  216 Now, the linuxrc process may still access the old root via its
  217 executable, shared libraries, standard input/output/error, and its
  218 current root directory. All these references are dropped by the
  219 following command:
  220 
  221 # exec chroot . what-follows <dev/console >dev/console 2>&1
  222 
  223 Where what-follows is a program under the new root, e.g. /sbin/init
  224 If the new root file system will be used with devfs and has no valid
  225 /dev directory, devfs must be mounted before invoking chroot in order to
  226 provide /dev/console.
  227 
  228 Note: implementation details of pivot_root may change with time. In order
  229 to ensure compatibility, the following points should be observed:
  230 
  231  - before calling pivot_root, the current directory of the invoking
  232    process should point to the new root directory
  233  - use . as the first argument, and the _relative_ path of the directory
  234    for the old root as the second argument
  235  - a chroot program must be available under the old and the new root
  236  - chroot to the new root afterwards
  237  - use relative paths for dev/console in the exec command
  238 
  239 Now, the initrd can be unmounted and the memory allocated by the RAM
  240 disk can be freed:
  241 
  242 # umount /initrd
  243 # blockdev --flushbufs /dev/ram0    # /dev/rd/0 if using devfs
  244 
  245 It is also possible to use initrd with an NFS-mounted root, see the
  246 pivot_root(8) man page for details.
  247 
  248 Note: if linuxrc or any program exec'ed from it terminates for some
  249 reason, the old change_root mechanism is invoked (see section "Obsolete
  250 root change mechanism").
  251 
  252 
  253 Usage scenarios
  254 ---------------
  255 
  256 The main motivation for implementing initrd was to allow for modular
  257 kernel configuration at system installation. The procedure would work
  258 as follows:
  259 
  260   1) system boots from floppy or other media with a minimal kernel
  261      (e.g. support for RAM disks, initrd, a.out, and the Ext2 FS) and
  262      loads initrd
  263   2) /linuxrc determines what is needed to (1) mount the "real" root FS
  264      (i.e. device type, device drivers, file system) and (2) the
  265      distribution media (e.g. CD-ROM, network, tape, ...). This can be
  266      done by asking the user, by auto-probing, or by using a hybrid
  267      approach.
  268   3) /linuxrc loads the necessary kernel modules
  269   4) /linuxrc creates and populates the root file system (this doesn't
  270      have to be a very usable system yet)
  271   5) /linuxrc invokes pivot_root to change the root file system and
  272      execs - via chroot - a program that continues the installation
  273   6) the boot loader is installed
  274   7) the boot loader is configured to load an initrd with the set of
  275      modules that was used to bring up the system (e.g. /initrd can be
  276      modified, then unmounted, and finally, the image is written from
  277      /dev/ram0 or /dev/rd/0 to a file)
  278   8) now the system is bootable and additional installation tasks can be
  279      performed
  280 
  281 The key role of initrd here is to re-use the configuration data during
  282 normal system operation without requiring the use of a bloated "generic"
  283 kernel or re-compiling or re-linking the kernel.
  284 
  285 A second scenario is for installations where Linux runs on systems with
  286 different hardware configurations in a single administrative domain. In
  287 such cases, it is desirable to generate only a small set of kernels
  288 (ideally only one) and to keep the system-specific part of configuration
  289 information as small as possible. In this case, a common initrd could be
  290 generated with all the necessary modules. Then, only /linuxrc or a file
  291 read by it would have to be different.
  292 
  293 A third scenario are more convenient recovery disks, because information
  294 like the location of the root FS partition doesn't have to be provided at
  295 boot time, but the system loaded from initrd can invoke a user-friendly
  296 dialog and it can also perform some sanity checks (or even some form of
  297 auto-detection).
  298 
  299 Last not least, CD-ROM distributors may use it for better installation
  300 from CD, e.g. by using a boot floppy and bootstrapping a bigger RAM disk
  301 via initrd from CD; or by booting via a loader like LOADLIN or directly
  302 from the CD-ROM, and loading the RAM disk from CD without need of
  303 floppies. 
  304 
  305 
  306 Obsolete root change mechanism
  307 ------------------------------
  308 
  309 The following mechanism was used before the introduction of pivot_root.
  310 Current kernels still support it, but you should _not_ rely on its
  311 continued availability.
  312 
  313 It works by mounting the "real" root device (i.e. the one set with rdev
  314 in the kernel image or with root=... at the boot command line) as the
  315 root file system when linuxrc exits. The initrd file system is then
  316 unmounted, or, if it is still busy, moved to a directory /initrd, if
  317 such a directory exists on the new root file system.
  318 
  319 In order to use this mechanism, you do not have to specify the boot
  320 command options root, init, or rw. (If specified, they will affect
  321 the real root file system, not the initrd environment.)
  322   
  323 If /proc is mounted, the "real" root device can be changed from within
  324 linuxrc by writing the number of the new root FS device to the special
  325 file /proc/sys/kernel/real-root-dev, e.g.
  326 
  327   # echo 0x301 >/proc/sys/kernel/real-root-dev
  328 
  329 Note that the mechanism is incompatible with NFS and similar file
  330 systems.
  331 
  332 This old, deprecated mechanism is commonly called "change_root", while
  333 the new, supported mechanism is called "pivot_root".
  334 
  335 
  336 Resources
  337 ---------
  338 
  339 [1] Almesberger, Werner; "Booting Linux: The History and the Future"
  340     ftp://icaftp.epfl.ch/pub/people/almesber/booting/bootinglinux-current.ps.gz
  341 [2] newlib package (experimental), with initrd example
  342     ftp://icaftp.epfl.ch/pub/people/almesber/misc/newlib-linux/
  343 [3] Brouwer, Andries; "util-linux: Miscellaneous utilities for Linux"
  344     ftp://ftp.win.tue.nl/pub/linux-local/utils/util-linux/

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